1. Field of the Invention
The invention relates to 2 and 3-dimensionally periodic porous structures and composites with improved properties incorporating the advantages of porous silicon and photonic bandgap materials. The inventive material, referred to as silicon nanofoam, shows enhanced and spectrally controlled, tunable photoluminescence and electroluminesce and finds use as high-luminosity light emitting diodes (LEDs), wavelength division multiplexors, high-active-area catalyst supports, photonic bandgap lasers, silicon-based UV detectors, displays, gas sensors, and the like.
2. Description of Related Art
Photonic bandgap materials, also referred to as photonic crystals, and the possibility to utilize them to manipulate light was introduced first by Yablonovitch, Phys. Rev. Lett. 58, 2059–2062 (1987) “Inhibited spontaneous emission in solid-state physics and electronics” and John; Phys. Rev. Lett. 58, 2486–2489 (1987); “Strong localization of photons in certain disordered dielectric superlattices”. The refractive index in photonic bandgap materials is periodically modulated creating a 3-D diffraction grating for electromagnetic radiation. Similar to electron interaction with the atomic potential in crystalline solids, photons interact with a spatially modulated dielectric medium, resulting in a series of exciting phenomena, like reflection stop bands, due to Bragg diffraction. If the contrast between alternating areas of high and low refractive index is large enough, the stop bands may overlap forming a forbidden frequency range, referred to as a photonic bandgap, where electromagnetic waves are not allowed to propagate in any direction inside the material. Therefore the photonic crystals can be viewed as photonic analogs to semiconductors with the electrons substituted by photons as information carriers. This analogy is related to the inhibition of spontaneous emission and light localization. Porous Silicon was first been produced in 1956; see Uhlir A., Bell System Tech. J., 35, (1956), 333. Recently it has become one of the most attractive nanomaterials because of its room-temperature efficient photoluminescence; see L. T. Canham, Appl. Phys. Lett. 57 (1990) 1046. The availability of luminescent silicon would allow creating all-silicon optoelectronic systems compatible with almost all products of the microelectronic industry. The current porous silicon LEDs use only a small fraction of the material due to power dissipation losses, limited active surface area, and non-uniform pore size distribution.
Photoluminescence in porous silicon is explained with quantum confinement of charges and excitons in the nanostructures created by the electrochemical etching of bulk silicon. Periodic photonic crystals are emerging as a basis for optoelectronics due to confinement and localization of photons. Silicon photonic crystals are among the best photonic crystals, showing wide photonic band gap due to high refractive index contrast. The first demonstrations of photonic bandgap in the microwave and infrared bands have been performed in bulky silicon photonic crystals. It has now been found that a new material, a silicon nanofoam, couples the advantages of the porous silicon and photonic crystals. Such photonic crystals are actively emitting photonic materials, with the nanoporosity being responsible for the emission of light, and the geometry (periodicity and defects) of the photonic bandgap structure controlling the propagation of the emitted photons. The existing porous silica devices however suffer from the problems of non-uniform pore size distribution and limited active surface area. These problems result in lowered photoluminescence intensity and considerable variability in this intensity from device to device.
Hexagonally packed arrays of silica (artificial opals) or latex microspheres (colloidal crystals) provide systems for observing the properties of photonic crystals. In order to achieve a high refractive index contrast, face-centered cubic (f.c.c.) ordered arrays may be used as templates to create inverse opal structures, comprising spherical cavities in a metallic or dielectric medium. This may be done by sintering of the template to form narrow “necks” between the spheres, synthesis or infiltration of another material inside the void space of the template, and removal of the f.c.c. matrix by chemical etching of the opal or by burning of the colloidal crystal. As a result one obtains a replica of the template, referred to as inverse opal. Inverse opals have been fabricated from metals, oxides, various forms of carbon, polymers, and semiconductors. The structures have applications as photonic crystals for the infrared region.
Silicon is a very attractive material for fabrication of infrared photonic crystals because it is transparent in the frequency range of interest for optical communications (1.25 μm–1.55 μm) and has refractive index of approximately 3.45 that is larger than the critical value of 2.8 required for opening of a photonic bandgap in a f.c.c. lattice of spherical cavities. Lin, S. Y. et al., IEEE J. Lightware Technol. 17, 1944–1947 (1999); “Three-dimensional optical photonic crystal” and Noda, S. et al., IEEE J. Lightware Technol. 17, 1948–1955 (1999) “A three-dimensional optical photonic crystal” have reported the fabrication of silicon optical PCs by lithographic techniques. However, these methods are complex and expensive and the obtained structures consist of only a few layers. Bertone, J., et al, Phys. Rev. Lett. 83, 300–33 (1999) “Thickness dependence of the optical properties of ordered silica-air and air-polymer photonic crystals” have shown that the photonic properties depend on the thickness of the structure with the stop bands becoming more pronounced as the number of layers increases. Blanco, A., et al, Nature 405, 437–439 (2000) “Large-scale synthesis of a silicon photonic crystal with a complete three-dimensional bandgap near 1.5 micrometers” reported the first demonstration of an infrared silicon photonic crystal with photonic bandgap centered at 1.46 μm. Chomski, E., et al., Chem. Vap. Dep. 2, 8–13 (1996) “New forms of luminescent silicon: silicon-silica composite mesostructures” and Dag, O., et al., Adv. Mater. 11, 474–480 (1999) “Photoluminescent silicon clusters in oriented hexagonal mesoporous silica film” have reported grown silicon inside the voids of bulk opals with particle size of 1.2 μm by chemical vapor deposition (CVD) using disilane (Si2H6) gas as a precursor. The best photonic properties have been achieved by surface infiltration, i.e. by forming thin silicon films around silica balls. The inverse opal obtained after removal of the opal template consists of hexagonally packed silicon spherical shells.
It has now been found that an improvement may be achieved by optimization of the CVD-based silicon deposition process. A low-pressure chemical vapor deposition (LPCVD) procedure allows the fabrication of both 2-D and 3-D silicon inverse opal photonic crystals with variable filling factor and size of the spherical pores on a nanometer scale. Thus, the range of tunability of the photonic properties is extended from the infrared to visible light frequencies. This invention thus provides a silicon inverse opal with photonic properties in the range of visible light which may form high-luminosity silicon nanofoam-based LEDs. The nanoporosity is responsible for the emission of light, and the periodic macroporosity of the photonic crystal structure controls the propagation of the emitted photons. Compared to conventional porous silicon, the inventive material has much larger active surface area since the whole volume of the material is used in the process for creating nanoporosity. The nanoporosity is created on the device after the periodic macroporosity has been created. This is unlike the process for the formation of conventional porous silicon via various chemical etching processes where only the surface of the bulk silicon is exposed to an etchant. Thus, the photoluminescence in these silicon nanofoams is enhanced by about ten-fold over conventional porous silicon. In addition, by tuning the position of the photonic band gap of the photonic crystal structure, even further photoluminescence enhancement at particular wavelength bands of the photoluminescence spectrum may be achieved due to nonlinear effects at the photonic band gap edges. The invention provides very low density nanofoams, which are highly periodic (photon confinement in photonic bandgap) and have nanoscale porosity. The invention combines the advantages of porous silicon and silicon-based photonic crystals and can be used to produce light emitting photonic crystals that exhibit two-levels of porosity: periodic microporosity in the silicon inverse opal backbone and random nanoporosity obtained by making the silicon backbone porous by chemical or electrochemical etching.